/*
 * Original Author -> Harry Yang (taketoday@foxmail.com) https://taketoday.cn
 * Copyright © TODAY & 2017 - 2022 All Rights Reserved.
 *
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS HEADER
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see [http://www.gnu.org/licenses/]
 */

package cn.taketoday.expression.spel.ast;

import cn.taketoday.expression.TypedValue;
import cn.taketoday.expression.spel.SpelEvaluationException;
import cn.taketoday.expression.spel.SpelMessage;
import cn.taketoday.lang.Nullable;

/**
 * Represents a reference to a value.  With a reference it is possible to get or set the
 * value. Passing around value references rather than the values themselves can avoid
 * incorrect duplication of operand evaluation. For example in 'list[index++]++' without
 * a value reference for 'list[index++]' it would be necessary to evaluate list[index++]
 * twice (once to get the value, once to determine where the value goes) and that would
 * double increment index.
 *
 * @author Andy Clement
 * @since 4.0
 */
public interface ValueRef {

  /**
   * Returns the value this ValueRef points to, it should not require expression
   * component re-evaluation.
   *
   * @return the value
   */
  TypedValue getValue();

  /**
   * Sets the value this ValueRef points to, it should not require expression component
   * re-evaluation.
   *
   * @param newValue the new value
   */
  void setValue(@Nullable Object newValue);

  /**
   * Indicates whether calling setValue(Object) is supported.
   *
   * @return true if setValue() is supported for this value reference.
   */
  boolean isWritable();

  /**
   * A ValueRef for the null value.
   */
  class NullValueRef implements ValueRef {

    static final NullValueRef INSTANCE = new NullValueRef();

    @Override
    public TypedValue getValue() {
      return TypedValue.NULL;
    }

    @Override
    public void setValue(@Nullable Object newValue) {
      // The exception position '0' isn't right but the overhead of creating
      // instances of this per node (where the node is solely for error reporting)
      // would be unfortunate.
      throw new SpelEvaluationException(0, SpelMessage.NOT_ASSIGNABLE, "null");
    }

    @Override
    public boolean isWritable() {
      return false;
    }
  }

  /**
   * A ValueRef holder for a single value, which cannot be set.
   */
  class TypedValueHolderValueRef implements ValueRef {

    private final TypedValue typedValue;

    private final SpelNodeImpl node;  // used only for error reporting

    public TypedValueHolderValueRef(TypedValue typedValue, SpelNodeImpl node) {
      this.typedValue = typedValue;
      this.node = node;
    }

    @Override
    public TypedValue getValue() {
      return this.typedValue;
    }

    @Override
    public void setValue(@Nullable Object newValue) {
      throw new SpelEvaluationException(
              this.node.getStartPosition(), SpelMessage.NOT_ASSIGNABLE, this.node.toStringAST());
    }

    @Override
    public boolean isWritable() {
      return false;
    }
  }

}
